1. Field of the Invention
This invention relates to a heat-reflecting pane or panel comprising a substantially transparent glass substrate, for example of soda-lime-silicate glass, and further comprising two layers of TiO.sub.2. The outside TiO.sub.2 layer comprises TiO.sub.2 in the rutile form with the intermediate layer TiO.sub.2 in the anatase form. Both TiO.sub.2 layers are formed in a similar manner by vapor deposition in vacuo and subsequent oxidation of the layer thus formed.
2. Description of the Prior Art
Heat-reflecting window glass panes of this kind in which a heat-reflecting TiO.sub.2 layer is applied by vapor-coating of a titanium layer in vacuo and subsequent oxidation of such layer at very high temperatures in air are known and have been described e.g., in a publication by G. Hass entitled "Preparation, Properties and Optical Applications of Thin Films of Titanium Dioxide" (G. Hass, Vacuum, Vol. 2, No. 4, pp. 331-345 (1952)). Depending upon the conditions in which the Ti layer is vapor-coated in vacuo, there are two TiO.sub.2 forms which may be produced in the subsequent oxidation of the Ti layer. When the titanium is vapor-coated rapidly in a good vacuum i.e., of approximately 10.sup.-5 mm Hg or even harder, the rutile form is produced, whereas if the vapor-coating proceeds relatively slowly in a less hard vacuum, e.g., of approximately 10.sup.-4 mm Hg, the anatase form is produced. TiO.sub.2 layers produced in this way have a variety of uses and optics to coat glass panes, e.g., as color filters and as sunshine-reflecting coatings, the thickness of the layer being made such as to act as a highly effective quarter-wave interference layer, over the spectral range for which a modification of the reflection properties of the substrate is required.
One particular purpose for which heat-reflecting glass panes of the kind described are used is in facade elements or infilling panels for buildings. The requirements in the case of infilling panels of this kind are for TiO.sub.2 -coated glass panels which have a high neutral reflection, possibly with a slight blue or yellow tint, in the visible spectral range. The usual practice with panels of this kind is for the TiO.sub.2 interference layer to be disposed on the outside of the building, the back of the glass substrate being treated with an opaque enamel or varnish, so that parts of the building behind the panel cannot be seen through it from the outside.
Films of TiO.sub.2 in the rutile form are very advantageous, more particularly for the last mentioned purpose, since such layers have a higher index of refraction than anatase layers and can therefore provide higher reflection values which are very desirable in the case of facade elements and infilling panels. It has also been found that rutile films are much harder and much more resistant to abrasion than anatase films. Consequently, panes of glass where the TiO.sub.2 interference film on the outside of the building has a rutile form can be exposed directly to the atmosphere without damage for a prolonged period of time. Also, the usual cleaning agents for outside surfaces of glass can be used to clean such panes or panels.
In some of the uses of TiO.sub.2 -coated glass panes or panels of the kind described, more particularly for use as a facade element or infilling panel, the glass must be toughened to comply with safety regulations. Toughening is necessary when infilling panels having rutile films are enameled on the back. Because of the presence of the enamel coating, which is impervious to radiation, the glass may become so hot in sunshine that heat cracks would occur in an untoughened glass substrate. The glass is prestressed or toughened in a known manner by being heated beyond its transformation temperature, the temperatures at which softening begins, and then cooled very abruptly. Temperatures of from about 570.degree. to 620.degree. C. are necessary for this purpose in the case of soda-lime-silicate glasses having the chemical composition of conventional flat glass.
The heating of the glass to toughen it in the manner described above may also be used to promote oxidation of the vacuum, vapor-coated Ti film. Alternatively, the vapor-coated Ti layer can first be oxidized at a temperature of from 400.degree. to 500.degree. C., in one furnace, the pane with its TiO.sub.2 coating being thereafter cooled and being subsequently heated again, for toughening, in another furnace to the temperature necessary to toughen the glass, between 570.degree. to 620.degree. C. in the case of soda-lime-silicate glass.
Theoretically, of course, oxidation of the Ti film to TiO.sub.2 should be carried out very rapidly to provide an acceptable layer of TiO.sub.2 in the rutile form. It is known from G. Hass, Vacuum Vol. 2, No. 4, p. 335, FIG. 3, that oxidation proceeds faster in proportion as the oxidation temperature is higher. Unfortunately, when in the known process the oxidizing temperature is increased to above 550.degree. C. in order to promote rapid oxidation, alterations occur in the rutile layers, which turn matt and dull and cause so much light scattering both in transmission and reflection that panes having such layers cannot be used for the purposes mentioned, that is as infilling panels and facade elements. A peculiarity is that these changes in the film occur only in rutile films. When other vacuum vapor-coating conditions are used, more particularly a softer vacuum and/or longer vapor-coating time, oxidation leading in the manner described to TiO.sub.2 layers having the anatase form and panes which have been coated in this way can be heated to relatively high temperatures, such as 550.degree. C. or more, without experiencing any of these changes in their coatings. Of course, the same difficulties of the rutile coatings undergoing changes always occur when the glass panes are toughened by heat treatment in the manner described, since for this purpose, and as already stated, the temperatures required are above 550.degree. C. and in the case of soda-lime-silicate glasses are preferably of from 570.degree. to 620.degree. C. These unwanted changes in the coating occur when the panes are heated to the temperatures required for toughening irrespective of whether oxidation of the Ti layer to TiO.sub.2 and heating to the glass toughening temperature are effected in a single step or whether the Ti layers are first oxidized at a relatively low temperature below 550.degree. C. and only subsequently, possibly after further processing, are the panes heated to the temperatures necessary for toughening.